The mail::Header family of
classes provide a convenient way for creating E-mail headers.
The usual approach is to instantiate a mail::Header::list(3x)
object, then use the “<<” operator repeatedly with any
combination of mail::Header::addresslist(3x),
mail::Header::encoded(3x),
mail::Header::mime(3x) and
mail::Header::plain(3x)
objects, which format different kinds of E-mail headers.
Finally, mail::Header::list(3x)
std::string converts the headers
to text, and returns a single string containing
nicely-formatted headers.